Continental Drift
Dana Desonie, Ph.D.
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Printed: September 6, 2013
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C ONCEPT
Concept 1. Continental Drift
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Continental Drift
• Identify the evidence Wegener had in support of his continental drift hypothesis.
• Apply the steps of scientific method to Wegener’s scientific investigation.
"Doesn’t the east coast of South America fit exactly against the west coast of Africa, as if they had once been
joined? This is an idea I’ll have to pursue." - Alfred Wegener to his future wife, December, 1910.
We can’t really get into Alfred Wegner’s head, but we can imagine that he started his investigations by trying to
answer this question: Why do the continents of Africa and South America appear to fit together so well? Is it an
accident that they do, or is there some geological reason?
Wegener’s Idea
Alfred Wegener, born in 1880, was a meteorologist and explorer. In 1911, Wegener found a scientific paper that
listed identical plant and animal fossils on opposite sides of the Atlantic Ocean. Intrigued, he then searched for and
found other cases of identical fossils on opposite sides of oceans. The explanation put out by the scientists of the
day was that land bridges had once stretched between these continents.
Instead, Wegener pondered the way Africa and South America appeared to fit together like puzzle pieces. Other
scientists had suggested that Africa and South America had once been joined, but Wegener was the idea’s most
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dogged supporter. Wegener amassed a tremendous amount of evidence to support his hypothesis that the continents
had once been joined.
Imagine that you’re Wegener’s colleague. What sort of evidence would you look for to see if the continents had
actually been joined and had moved apart?
Wegener’s Evidence
Here is the main evidence that Wegener and his supporters collected for the continental drift hypothesis:
• The continents appear to fit together.
• Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but are on
continents that are now widely separated (Figure 1.1). Wegener proposed that the organisms had lived side
by side, but that the lands had moved apart after they were dead and fossilized. His critics suggested that the
organisms moved over long-gone land bridges, but Wegener thought that the organisms could not have been
able to travel across the oceans.
– Fossils of the seed fern Glossopteris were too heavy to be carried so far by wind.
– Mesosaurus was a swimming reptile, but could only swim in fresh water.
– Cynognathus and Lystrosaurus were land reptiles and were unable to swim.
FIGURE 1.1
Wegener used fossil evidence to support
his continental drift hypothesis. The fossils of these organisms are found on lands
that are now far apart.
• Identical rocks, of the same type and age, are found on both sides of the Atlantic Ocean. Wegener said the
rocks had formed side by side and that the land had since moved apart.
• Mountain ranges with the same rock types, structures, and ages are now on opposite sides of the Atlantic
Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges
in eastern Greenland, Ireland, Great Britain, and Norway (Figure 1.2). Wegener concluded that they formed
as a single mountain range that was separated as the continents drifted.
• Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the
equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most
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Concept 1. Continental Drift
FIGURE 1.2
The similarities between the Appalachian and the eastern Greenland mountain ranges are evidences for the
continental drift hypothesis.
of the Earth. Today, glaciers only form on land and nearer the poles. Wegener thought that the glaciers
were centered over the southern land mass close to the South Pole and the continents moved to their present
positions later on.
• Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal
seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these
creatures were alive in warm climate zones and that the fossils and coal later drifted to new locations on the
continents. An animation showing that Earth’s climate belts remain in roughly the same position while the
continents move is seen here: http://www.scotese.com/paleocli.htm.
• Wegener thought that mountains formed as continents ran into each other. This got around the problem of the
leading hypothesis of the day, which was that Earth had been a molten ball that bulked up in spots as it cooled
(the problem with this idea was that the mountains should all be the same age and they were known not to be).
An animation showing how the continents split up can be found here: http://www.exploratorium.edu/origins
/antarctica/ideas/gondwana2.html.
Summary
• Alfred Wegener did some background reading and made an observation.
• Wegener then asked an important question and set about to answer it.
• He collected a great deal of evidence to support his idea. Wegener’s evidence included the fit of the continents,
the distribution of ancient fossils, the placement of similar rocks and structures on the opposite sides of oceans,
and indicators of ancient climate found in locations where those climates do not exist today.
Practice
Use these resources to answer the questions that follow.
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MEDIA
Click image to the left for more content.
1. Who was Alfred Wegener?
2. What evidence did Wegener find for Pangaea?
3. What was the response to Wegener’s hypothesis?
MEDIA
Click image to the left for more content.
4. What is the continental drift hypothesis?
5. What do the continental plates consist of?
6. What were formed when Pangaea broke apart?
7. How long ago did the continents reach their present position?
Review
1. How did Wegener become interested in the idea that continents could move?
2. What did he need to do to explore the question and make it into a reasonable hypothesis?
3. How did Wegener use fossil evidence to support his hypothesis?
4. How did Wegener use climate evidence from rocks to support his hypothesis?
References
1. Courtesy of the US Geological Survey, Osvaldocangaspadilla. . Public Domain
2. Appalachian mountain image copyright Geir Olav Lyngfjell, 2010 and eastern Greenland mountain image
copyright Tomas Cereda, 2010. . Both images used under licenses from Shutterstock.com
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